The sulfide ion is S-2, an anion with a -2 charge on it. In aqueous solutions, sulfide ions are only present in large concentrations at alkalinepH (high pH), because at lower pH, H+ will combine with sulfide ions to form HS- or H2S. HS- is the hydrogen sulfide ion, hydrosulfide ion, or sulfhydryl ion. H2S is hydrogen sulfide, a water-solublegas which is a weak diprotic acid. Ionically bonded sulfides can be thought of as salts of the acid hydrogen sulfide. Many inorganic sulfide salts are not very water-soluble and many have very low solubility in water. If an -SHfunctional group is covalently bonded to another atom or group such as an organic radical R in a thiol, then it is typically called a sulfhydryl group. Such sulfhydryl groups can also be weakly acidic, and can give up an H+ to form a substituted sulfide ion. For example, ethyl hydrosulfide, C2H5SH, can give up an H+ to form an ethyl sulfide ion,C2H5S-, although often other names are used for such compounds; see Thiol.

The sulfur in sulfides (or in the sulfide functional groups) is in its lowest oxidation state. In sulfides, this sulfur can often be oxidized to a higher oxidation state. For example, dimethyl sulfide (CH3-S-CH3) could be oxidized to dimethyl sulfoxide (CH3-SO-CH3), which could in turn be oxidized to dimethyl sulfone (CH3-SO2-CH3). Disulfides are similar compounds having two sulfur atoms covalently bonded together and covalently or ionically bonded to the rest of the molecule or compound.

Safety

Many sulfides are significantly toxic by inhalation or injection, especially if the metal ion is toxic. Additionally many sulfides, when exposed to a strong mineral acid, will release toxic hydrogen sulfide - and this includes your stomach acids!

Also, many sulfides are somewhat flammable, and a few are highly flammable. When a sulfide burns, the fumes usually include toxic sulfur dioxide (SO2) gas.